Trapezium (Theta1Orionis)

Trapezium (Theta1Orionis) is a star cluster at the heart of the Orion Nebula consisting of more than 1,000 young stars crowded into a space about four
light-years in diameter (roughly the distance between the Sun and the next
nearest star). A few of these stars are hot O and B stars. The majority, however, are later-type,
cooler, less luminous, and less massive pre-main sequence stars of a solar
mass or less.

With an average stellar age of only one million years, the Trapezium is
one of the youngest clusters known. Most of its member stars are hidden
by dust or by the glare of the nebula but are visible at infrared wavelengths.

The four brightest stars (A, B, C, and D), which form the vertices of a
trapezium and give the cluster its name, can be seen easily with a small
telescope. Two fainter stars of eleventh magnitude, E and F, show up in
moderately sized amateur instruments under good seeing conditions. A further
two, of 16th magnitude, G and H, are only visible in very large amateur
scopes.

Theta1 Orionis A, also known as V1016, is an eclipsing
binary with a period of 65.432 days and a magnitude range of 6.72 to
7.65. Theta1 B, also known as BM Orionis, is another eclipsing
binary, spectral type B2-B3, with a period of 6.471 days and a magnitude
range of 7.90 to 8.65. An infrared companion has also been discovered in
the Theta1 A system, making it a triple star, and Theta1 B is now known to be a quadruple system, with three components detected
separately in the near-infrared. Infrared observations have that shown Theta1 C, which is actually the brightest star in the cluster, is a close binary
as well.